The present invention relates generally to the field of methods of identification or detection. In particular, the present invention relates to methods of identification or detection utilizing photoluminescent compositions containing photoluminescent phosphorescent materials and photoluminescent fluorescent materials whose emission signature lies partly or fully in the infrared region of the electromagnetic spectrum. As well, the invention relates to methods of identification or detection utilizing photoluminescent compositions which are high in intensity and high in persistence. The present invention also relates to objects containing the photoluminescent compositions.
Photoluminescent materials and compositions that contain photoluminescent phosphorescent materials with emissions in the visible region of the electromagnetic spectrum have been disclosed. For example, metal sulfide pigments which contain various elemental activators, co-activators and compensators have been prepared which absorb at 380-400 nm and have an emission spectrum of 450-520 nm. Further examples of sulfide photoluminescent phosphorescent materials that have been developed include CaS:Bi, which emits violet blue light; CaStS:Bi, which emits blue light; ZnS:Cu, which emits green light; and ZnCdS:Cu, which emits yellow or orange light.
The term “persistence” of phosphorescence is generally a measure of the time, after discontinuing irradiation that it takes for phosphorescence of a sample to decrease to the threshold of eye sensitivity. The term “long-persistent phosphor” historically has been used to refer to ZnS:Cu, CaS:Eu,Tm and similar materials which have a persistence time of only 20 to 40 minutes.
Recently, phosphorescent materials that have significantly higher persistence, up to 12-16 hours, have been reported. Such phosphors generally comprise a host matrix that can be alkaline earth aluminates (oxides), an alkaline earth silicate, or an alkaline earth alumino-silicate.
Such high luminous intensity and persistence phosphors can be represented for example, by MAl2O3 or MAl2O4 wherein M can comprise a plurality of metals at least one of which is an alkaline earth metal such as calcium, strontium, barium and magnesium. These materials generally deploy Europium as an activator and can additionally also use one or more rare earth materials as co activators. Examples of such high intensity and high persistence phosphors can be found, for example, in patents U.S. Pat. Nos. 5,424,006, 5,885,483, 6,117,362 and 6,267,911 B1.
High intensity and high persistence silicates have been reported in U.S. Pat. No. 5,839,718, such as SrBaO.MgMO.SiGe:EuLn wherein M is beryllium, zinc or cadmium and Ln is chosen from the group consisting of the rare earth materials, the group 3A elements, scandium, titanium, vanadium, chromium, manganese, yttrium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, indium, thallium, phosphorous, arsenic, antimony, bismuth, tin, and lead.
Photoluminescent compositions comprising only phosphorescent materials with emissions in the infrared region have been reported. Such phosphorescent materials consist of doped ZnCdS. These materials have been shown to have observable tail emissions into the visible region and consequently would not have utility for clandestine markings. The reported use of these phosphors has been as a “laminated panel of the infrared phosphor powder” and have not been formulated into a composition containing other materials. As previously mentioned, ZnS based phosphors have afterglow characteristics significantly inferior to aluminate photoluminescent pigments, particularly alkaline earth aluminate oxides. It is not surprising therefore that such materials or the laminated panels made therefrom have neither been used for clandestine detection or for detection applications wherein activation and detection can be decoupled spatially and temporally.
Photoluminescent compositions which contain combinations of ZnS phosphorescent materials and fluorescent materials have also been disclosed. However the use of these fluorescent materials has been limited to either altering the charging (activating) radiation or altering the visible daylight or emission color. Since the absorbance spectrum of ZnS phosphorescent materials are primarily in the long UV and blue regions of the electromagnetic spectrum, attaining reasonable afterglow requires downshifting some of the incident natural radiation with fluorescent materials. Use of ZnS with fluorescent materials is generally limited to altering the color observed in daylight. Furthermore the fluorescent materials described exist as aggregates, that is, they are not molecularly dispersed in the polymer resin, consequently resulting in low emission efficiencies.
Photoluminescent compositions have also been contemplated which contain a series of fluorescent materials. One of the fluorescent materials absorbs and emits radiation which is then absorbed by a companion fluorescent material which then emits radiation to give a final infrared emission. As can be appreciated, use of fluorescent materials does not provide for any continued emission once the absorbable radiation is removed. These compositions have no provision for continued emission of infrared radiation that can be detected at a future time, that is, after activation has ceased. The need for activating the materials immediately prior to detection will also require possession of activating equipment at site of detection thereby limiting flexibility and/or portability and thus will not permit stealth detection.
It can be seen then that prior efforts to develop photoluminescent compositions and particularly photoluminescent compositing containing both phosphorescent and fluorescent materials have been directed primarily at emissions in the visible region. Attention has not been given to photoluminescent compositions comprising both phosphorescent and fluorescent materials with emissions in the infrared region of the electromagnetic spectrum. Thus there is a need for photoluminescent compositions wherein emissions, partly or fully in the infrared region, continue after activation has ceased, that is, activation and detection are separated temporally. There is also a need for activation and detection to be separated spatially, that is, activation is not required at the time of detection, so that activating equipment is not required to be carried and be present at the time of detection. Development of photoluminescent compositions whose emissions are partly or fully in the infrared region and which are also of high intensity and persistence, will enable a high degree of spatial and temporal decoupling, that is, detection can occur at great distances from the object and also long after activation has ceased.
Although methods for uniquely marking and identifying objects have received thought and attention, such methods do not enable stealth detection. In many cases, such as, for example, identification of friendly forces in the combat theater, the identifying markings need to be unobservable by enemy personnel, or for example, in anti-counterfeit applications wherein, the identifying markings need to be hidden to avoid detectability of such markings by counterfeiters. Clandestine or stealth identification, wherein the emissions from the photoluminescent markings are not ordinarily observable by a human observer (without specific detection equipment), but detectable by friendly forces, and further wherein activation is not required during detection (such activation being potentially detectable by others), will be of high value in the combat theater for stealth detection of combat equipment, or personnel. Such markings will also be of value for stealth combat operations, or for covertly marking enemy targets for tracking or elimination.
An authentication and identification method based upon marking-up groups of microsized particles normally visible to the naked eye with each particle in each group being of selected uniform size, shape, and color has been proposed. Identification is established by transferring a population of particles from a selected number of the groups to the item to be identified, and then confirming by examining the marked item under high magnification which requires the magnifying device to be in close proximity to the item. It can be readily seen that such methods will have limitations in that one has to be in close proximity to the object to enable detection.
Another method includes incorporating into a carrier composition a mixture of at least two photochromic compounds that have different absorption maxima in the visible region of the electromagnetic spectrum. Authentication or identification requires activating the photochromic compounds immediately prior to detection and subsequently examining the display data. Such activation prior to detection does not allow for temporal decoupling, that is, an object can not be activated, moved and detected at a later time, nor can it be detected in a dark environment.
Other systems have been disclosed wherein items are marked with ink comprised of two or more fluorescent materials wherein the emission from one fluorescent dye is absorbed and reemitted by a second fluorescent dye and so forth in a daisy chain mechanism. The subsequent emissions can be down-shifted to the infrared region. As can be appreciated, a fundamental characteristic of fluorescent materials is that the emission immediately ends when the source of charging is removed. Thus authentication comprises activating or exciting the materials immediately prior to detection with an ultraviolet source, and then rapidly detecting the subsequent emission. When the activation source is removed identification ceases. Consequently activation and detection cannot be decoupled temporally. Thus, these detection methods will not enable stealth identification. Additionally, the activating equipment will have to be present at the time of detection and hence such methods will not allow for flexibility and portability during detection.
As can be seen from the above discussion, there is a need for detection methods using photoluminescent compositions which emit partly or fully in the infrared region of the electromagnetic spectrum. Furthermore there is also a need for photoluminescent materials and methods that enable the act of detection of the object to be decoupled spatially from the object and/or its activation source, that is, detection can occur away from the object and/or its activation source, and also wherein, detection can be decoupled temporally from activation, that is, detection can occur at a time later than the activation. It should be noted that decoupling of activation and detection also allows for flexibility and portability in the act of detection, allowing for clandestine or stealth identification.
It can be appreciated that for optimal luminescent performance, specific photoluminescent phosphorescent materials and mixtures of such materials need to be adapted for use in varying conditions, be it excitation conditions or environmental considerations. Water-resistant formulations suitable for protecting the photoluminescent ingredients and compositions that minimize photolytic degradation are sought-after. Beyond the selection of the photoluminescent materials it should be noted that the emission intensity and/or persistence from a photoluminescent composition is greatly affected by both the way in which the photoluminescent phosphorescent material is distributed and the additives used, as well as the manner in which that composition is applied.
The improper selection and use of composition materials, such as resins, dispersants, wetting agents, thickeners, and the like can diminish the emission intensity emanating from the composition. This can occur, for example, due to agglomeration or settling of photoluminescent phosphorescent ingredients, either during handling of the formulated materials or after application of the formulated materials. The reduction in emission intensity and/or persistence can result from both incomplete excitations and/or due to scattering of emitted radiation. The scattering of photoluminescent emissions can be either due to agglomeration of photoluminescent phosphorescent material or as a consequence of electromagnetic radiation scattering by one or more of the additives selected to stabilize the photoluminescent phosphorescent pigment dispersion. The net result will be lower emission intensity and/or persistence.
In general, the use of colorants in the form of pigments that are absorptive of visible electromagnetic radiation to impart daylight color to photoluminescent compositions, even when such colorants are not absorptive of photoluminescence, can result in degradation of photoluminescent intensity and/or persistence by virtue of either scattering of the photoluminescence or by inadequate charging of photoluminescent phosphorescent materials. Hence, while absorptive colorants can be used to alter both the daytime appearance of photoluminescent objects and the nighttime emission, such usage will result in a lowering of emission intensity and/or persistence. This is why a majority of compositions whose daylight color has been altered are rated for low intensity and/or persistence. Further, such usage also precludes the achievement of daytime colors and nighttime emissions being in the same family of colors. Identification, whether clandestine or not can also result from markings that have been rendered as stealth markings, that is, the daylight color of the photoluminescent markings can be formulated in such a manner that the markings blend in with the area surrounding the marking so as not to be distinguishable from the surrounding area.
Photoluminescent phosphorescent compositions utilizing various additives to allow dispersion, anti-settling and other compositional properties have been disclosed. These additives include alkyd resins and modified castor oil for rheology modification, synthetic cellulosic resin binders and silica-based powders used as suspending fillers, absorptive pigments as colorants for imparting daytime color, “crystalline fillers”, and secondary pigment particles. Compositions containing any of these additives, generally in a solid particulate state, by virtue of scattering phenomenon, can result in lower intensity and/or persistence of emissions from objects deploying them, as has been mentioned above.
It can therefore be seen from the above discussions that there is a need for stable photoluminescent compositions whose emission intensity is high and persistent, and whose emission is partly or fully in the infrared region of the electromagnetic spectrum, such emissions being suitable for methods of clandestine or stealth identification or otherwise identification or detection of objects, such methods designed to decouple activation and detection both spatially, e.g., at a distance away from the object to be detected and/or the activation device, and temporally, e.g., detection at a time later than the activation. In addition there is a need for portability of the detector used in identification or detection processes. Furthermore there is also a need for stealth markings wherein the marking is indistinguishable from its surroundings.